Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by IC engines. Conventional exhaust gas aftertreatment systems include any of several different components to reduce the levels of regulated exhaust emissions present in exhaust gas. For example, certain exhaust aftertreatment systems for diesel-powered IC engines include a selective catalytic reduction (SCR) catalyst to convert NOx (NO and NO2 in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O) in the presence of ammonia (NH3).
Generally in such conventional aftertreatment systems, an exhaust reductant (e.g., a diesel exhaust fluid such as urea) is injected into the aftertreatment system to provide a source of ammonia and to mix with the exhaust gas. When the exhaust reductant is in the form of a urea based solution, the reductant that can react with the NOx over the catalyst is formed through a complex processes (involving evaporation, thermolysis, decomposition). These processes are endothermic and generally require sufficient temperature (>200 degrees C.) to achieve a good yield. The reduction byproducts of the exhaust gas are then fluidically communicated to the catalyst included in the SCR aftertreatment system to decompose substantially all of the NOx gases into relatively harmless byproducts which are expelled out of such conventional SCR aftertreatment systems.
Some aftertreatment systems can also include a turbine disposed in a flow path of the exhaust gas. The turbine can be a part of a turbocharging system that extracts energy from the exhaust gas flowing through the turbine to operate an associated compressor. The exhaust reductant is generally injected into the exhaust flow after the exhaust gas has passed through the turbine.
If the exhaust reductant is injected into an environment of relatively low temperature (<200 degrees C.), undesirable side effects can occur. For example, deposits associated with the exhaust reductant can form leading to less efficient mixing of the exhaust reductant and loss of control fidelity, both of which can adversely impact the NOx conversion efficiency of the aftertreatment system. Additionally, the creation of deposits can increase the exhaust backpressure on the IC engine, adversely impacting engine performance. Temperatures downstream of the turbine tend to be lower than that upstream of the turbine.
Furthermore, future emission legislation is expected to require higher NOx conversion efficiencies at lower exhaust gas temperatures. For example, as fuel economy standards become more stringent, the expectation is that there will be less waste heat available in the exhaust gas. At the same time, criteria pollutant standard are also being tightened. The combination leads to the need to have a system that enables high aftertreatment system effectiveness at low exhaust gas temperatures. However, the exhaust gas thermal energy of currently available technologies does not support low temperature dosing of aftertreatment systems with the exhaust reductant.